The present invention relates to a data transferring system, more detailedly, to a data transferring system providing a plurality of devices which uses a bus and which devices are connected in series. The data transferring system in this invention includes, for example, a central processing apparatus, an input-output device, and memory devices which are connected to a common bus line. In this invention, the devices which use the bus correspond to the input-output devices and the memory device mentioned above.
In a data transferring system providing a plurality of devices which use a bus and which are connected in series, each device which uses the bus sends out a signal for requesting a transfer of data, and a unit for controlling the bus commonly provided to the devices which monitor the signal utilized for requesting the transfer of data and consequently sends out a signal for acknowledging the transfer of data to each of said plurality of devices.
In the above-mentioned data transferring system, each device which uses the bus independently sends out a signal for requesting the transfer of data; the unit for controlling the bus to be used detects the signal employed for requesting the transfer of data; and the unit sends out the acknowledging signal for transferring data to each of the devices. In addition, the priority order utilized in the transference of data among the above-mentioned devices is determined such that the device which is nearest to the above-mentioned unit is of the highest priority order. That is, the above-mentioned acknowledging signal for transferring data is sent from unit to one of the devices. When the device is sending out the signal for requesting the transfer of data, the device receives the acknowledging signal; however, the acknowledging signal is not sent to the next device which uses the bus. When the device does not send out a signal for requesting the transfer of data, the device transmits the acknowledging signal to the next device. As mentioned above, the device which has the lowest priority order and which is using the bus is placed at a position farthest from the device for controlling the bus.
Therefore, in the device which has a low priority order and which is frequently used, much time is required for receiving the acknowledging signal for transferring the data from the unit controlling the bus; and thereby the efficiency of the total system is decreased because of the increase in the waiting time.